inexpensive, easy to find alarms that rely on ionization technology. they work well to detect fires with fast flames, but experts say they may not save you in smoldering smoky fires that can strike while you sleep. don russell is a scientist at texas a&m. when i go to the store to buy a smoke detector, i assume that it's going to sound when there's smoke. >> that's a reasonable assumption, but it's wrong. >> reporter: we had him set up a test, placing three ionization detectors, the kind that most of us have, in a room, then setting a couch on fire. toxic smoke is building, but it takes 36 minutes for the first detector to go off. but there's another technology out there that gives you better warning in these fires. it's called a photoelectric detector. so dr. russell set up another test, this time with a photoelectric next to those three ionization detectors. 17 minutes in, with barely any smoke in the room, the photoelectric sounds the alarm. the ionizations? they're still silent for another 21 minutes. even with smoke everywhere. >> if i would have